The present invention generally relates to a windshield antenna.
Windshield antennas have been used for a variety of frequency ranges in vehicles, in particular, motor vehicles. They include an antenna base having a frequency-tuned (resonant) radiator glued to the outside of the window pane, in particular, a windshield, and a connecting module glued to the pane on the inside.
Signal transmission must take place between these two components through the pane.
In conventional windshield antennas, signals are transmitted through the pane capacitively using capacitor plates of different sizes. This coupling is particularly well-suited for frequencies from 140 kHz to approximately 1500 MHz. For signals above a frequency of 1500 MHz, capacitive coupling is ineffective since the capacitor plates themselves transmit through the pane. For low-loss coupling for signals having frequencies over 1500 MHz, slot antennas can be used as described in U.S. Pat. No. 5,451,966.
At low frequencies, the vehicle body, connected to the connecting module via a grounding strip, which should be as short as possible ( less than  less than xcex/4), can be used as a reference ground. From approximately 400 MHz, tuned counterweights referred to as radials are used, which are glued to the pane on both sides of the connecting module. For coupling with slot antennas (over approximately 1500 MHz) the reference ground surrounds the slot.
Conventional combination antennas with a screwed-on antenna base are also available. For example, Bosch offers antennas for DAB (Digital Audio Broadcast), i.e., band III +L-band, antennas for C networks and D networks, i.e., 450 MHz +900 MHz and other combinations.
If DAB radio programs are to be received with a windshield antenna, a combination antenna must be used in all cases, since DAB programs are transmitted in xe2x80x9cband III,xe2x80x9d i.e., between 174 and 240 MHz and xe2x80x9cL band,xe2x80x9d i.e., between 1452 and 1492 MHz.
Cable damping for common antenna cables used in motor vehicles is approximately 0.2 dB/m in band III and approximately 0.8 dB/m in the L band. To compensate for this damping, it is advisable, especially in the L band, to use an antenna amplifier. Presently an active diplexer in a dedicated housing located at a distance from the windshield antenna is used.
An object of the present invention is to provide a windshield antenna (also referred to as an on-glass antenna) having a simple design in which the capacitive transmitter and the respective circuit and connecting elements are implemented in a simple manner. Furthermore or as an alternative, the windshield antenna according to the present invention may be capable of transmitting at least one frequency band below, as well as at least one frequency band above, a certain frequency such as, for example, 1450 MHz.
Furthermore or as an alternative, according to the present invention, a DAB windshield antenna having multirange couplers is to be provided, which properly receives not only band III, but also the L band.
The present invention can be used, in particular, for receiving radio programs in a DAB format (Digital Audio Broadcast). DAB is divided into the frequency bands: band III, 174 to 240 MHz, and L band, 1452 to 1492 MHz. Other applications of the present invention are for antennas for analog broadcasting +E network or D network +E network, for example.
In an exemplary embodiment according to the present invention, an L band amplifier is provided in the connecting module; thereby not only is a very compact design achieved, but also the L band signals, particularly affected by cable damping, are effectively amplified.
In another embodiment according to the present invention, printed circuit boards are provided in the antenna base and the connecting module whose surfaces facing the pane, which are designed to be conductive, form the capacitive transmitter, and whose opposite surfaces carry connecting and/or circuit elements. Thereby not only a particularly inexpensive design but also particularly short signal paths are achieved.
To couple two frequency bands, for example, one frequency band below and another frequency band above 1450 MHz, which are to be sent to or received by a combination antenna, through a pane, a slot antenna for the upper frequency band is combined with a capacitive coupling for the lower frequency band in the windshield antenna. It is advantageous if the conductive surface surrounding the slot antenna is used as capacitor plates for coupling the lower frequency band, the antenna rod being connected to the coupling elements on one side and the internal conductor of the common coaxial connecting cable being connected to the coupling elements on the other side of the pane, via a high/low-pass combination.
The conductive surface surrounding the slot is used as a reference ground for the upper frequency band. A grounding strip ( less than  less than xcex/4) to the vehicle body is used for the lower frequency band at frequencies  less than 400 MHz; at frequencies between 400 and 1450 MHz tuned radials are used. The reference grounds are connected to the external conductor of the common coaxial connecting cable via a high/low-pass combination. In the simplest case, these high/low-pass combinations can be implemented by one coil and one capacitor each.
Furthermore, according to the present invention, the antenna signals from the antenna base are picked off behind the pane in the connecting module of a DAB windshield antenna. This is achieved using capacitor plates for both frequency ranges or a combination of a capacitor plate for band III and a slot antenna for the L band. These coupling elements can be designed as printed circuit boards without any additional expense. The electronic components used for a frequency diplexer and an antenna amplifier are then mounted on this printed circuit board.
Another advantage of the present invention is that a windshield antenna can be designed as a combination antenna for two frequency bands, one below and the other above 1450 MHz. Thus only one antenna and one connecting cable is needed for transmitting two frequency bands without the use for drilling a hole for the cable feed-through.
It is advantageous that when frequencies  less than 400 MHz and frequencies  greater than 1400 MHz (for example, DAB) are jointly transmitted, the view through the windshield is not negatively affected by glued-on radials.
Another advantage is that the connecting module of a windshield antenna can be equipped with an amplifier without using an additional component, i.e., an additional printed circuit board with mechanical brackets; this amplifier compensates for the damping of an antenna cable. In this manner, the additional costs for an antenna amplifier are minimized.
When the windshield antenna is used as a multirange antenna, it has a combination antenna rod which allows the reception of the desired frequency bands.